The use of non-selective silicon germanium (SiGe) epitaxy is one of the key processes in modern manufacturing technologies for bipolar transistors. Non-selective epitaxy defines the electrical properties of the bipolar base and makes high speed bipolar devices possible. During this non-selective deposition, single crystal SiGe is deposited on open silicon regions while poly SiGe is grown on oxide surfaces like shallow trench isolation structures (STI).
An example of an NPN transistor structure 10 manufactured by non-selective epitaxy is shown in FIG. 1. A collector C is formed as an epitaxial layer and etched so that shallow trench isolation structures STI are formed therein. After deposition of a gate poly layer G, the shallow trench isolation structures STI are etched. An epitaxial single crystal SiGe base layer B is deposited on the moat region M of the collector C and a poly SiGe gate layer BP is grown on the shallow trench isolation structures STI adjacent to and simultaneous with deposition of the base layer B. The emitter of this bipolar structure is not shown here.
However, because of the lattice mismatch between germanium and silicon, between the single crystal SiGe base layer B and the polysilicon gate regions BP, stress is induced in the epitaxial SiGe layers. This stress can result in defects such as dislocations.
It is an object of the invention to provide a bipolar transistor in which lattice defects are reduced or eliminated.